Your vehicle’s aerodynamics affects your fuel economy to varying degrees. While it may not be possible to change the aerodynamics of your vehicle, it is possible to minimize the impact.
Items like cargo containers and ski/bike racks create additional wind resistance that reduce your fuel efficiency. A study in France found that roof mounted accessories cost you between 7 and 35% depending on what you have on top of your vehicle. No matter what the size, it is costing you more!